USB: serial: remove generic disconnect callback

Remove the now empty generic disconnect callback and make the disconnect
callback non-mandatory.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/usb/serial/usb-serial.c b/drivers/usb/serial/usb-serial.c
index e7f97b5..569b679 100644
--- a/drivers/usb/serial/usb-serial.c
+++ b/drivers/usb/serial/usb-serial.c
@@ -1095,7 +1095,8 @@
 				device_del(&port->dev);
 		}
 	}
-	serial->type->disconnect(serial);
+	if (serial->type->disconnect)
+		serial->type->disconnect(serial);
 
 	/* let the last holder of this object cause it to be cleaned up */
 	usb_serial_put(serial);
@@ -1304,7 +1305,6 @@
 	set_to_generic_if_null(device, chars_in_buffer);
 	set_to_generic_if_null(device, read_bulk_callback);
 	set_to_generic_if_null(device, write_bulk_callback);
-	set_to_generic_if_null(device, disconnect);
 	set_to_generic_if_null(device, release);
 	set_to_generic_if_null(device, process_read_urb);
 	set_to_generic_if_null(device, prepare_write_buffer);